fix(vars): resolve wiki by index in the vimwiki compat shim (R15)

`vimwiki#vars#get_wikilocal(key, N)` previously ignored the wiki-index
argument and always read the scalar `g:nuwiki_*` globals (the first
wiki), so third-party plugins (vimwiki-sync, vim-zettel) got the wrong
wiki's path/ext/syntax in a multi-wiki setup.

Now:
- an explicit numeric index selects that wiki from `g:nuwiki_wikis[N]`,
  falling back per-key to the scalar globals;
- with no index, the wiki owning the current buffer is resolved (matching
  upstream's `g:vimwiki_current_idx` behaviour);
- an out-of-range index clamps to the first wiki.

The single-wiki shorthand path is unchanged. Verified with a headless
Vim test (by-index, global fallback, clamp, no-arg, shorthand) and the
Vim keymap harness (301/18/21, 0 failed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

autoload/vimwiki/vars.vim

@@ -6,6 +6,30 @@
 " Values are read from nuwiki's own config variables so users don't have
 " to duplicate settings.
 
+" Resolve which wiki a `get_wikilocal` call refers to, mirroring upstream:
+"   - an explicit numeric second argument selects that wiki (0-based);
+"   - otherwise the wiki owning the current buffer is used
+"     (upstream's `g:vimwiki_current_idx`);
+"   - failing that, the first/shorthand wiki (index 0).
+function! s:resolve_index(...) abort
+  if a:0 >= 1 && type(a:1) == type(0)
+    return a:1
+  endif
+  let l:file = expand('%:p')
+  if !empty(l:file)
+    let l:n = 0
+    for l:w in nuwiki#commands#wiki_list()
+      let l:wroot = fnamemodify(expand(l:w.root), ':p')
+      if l:wroot[len(l:wroot) - 1:] !=# '/' | let l:wroot .= '/' | endif
+      if a:file !=# '' && l:file[: len(l:wroot) - 1] ==# l:wroot
+        return l:n
+      endif
+      let l:n += 1
+    endfor
+  endif
+  return 0
+endfunction
+
 " Per-wiki local config values.
 "
 " Supported keys:
@@ -14,28 +38,37 @@
 "   ext / extension — file extension (default '.wiki')
 "   syntax          — syntax name (default 'vimwiki')
 "
-" LIMITATION: the optional second argument (upstream's wiki index) is
-" ignored — values always come from the scalar `g:nuwiki_*` vars, i.e.
-" the first/shorthand wiki. Third-party plugins that query a specific
-" wiki by index in a multi-wiki setup will get the first wiki's values.
-" The shim exists for single-wiki compatibility (vimwiki-sync, vim-zettel);
-" extend here with `g:nuwiki_wikis[a:1]` lookups if multi-wiki support is
-" needed.
+" The optional second argument is the wiki index (upstream's signature);
+" when omitted, the wiki owning the current buffer is used. Values are
+" pulled from the matching `g:nuwiki_wikis` entry, falling back to the
+" scalar `g:nuwiki_*` globals for keys that entry doesn't set.
 function! vimwiki#vars#get_wikilocal(key, ...) abort
+  if a:key ==# 'is_temporary_wiki'
+    return 0
+  endif
+
+  let l:idx = call('s:resolve_index', a:000)
+  let l:wikis = nuwiki#commands#wiki_list()
+  if l:idx < 0 || l:idx >= len(l:wikis)
+    let l:idx = 0
+  endif
+  let l:w = l:wikis[l:idx]
+
   if a:key ==# 'path'
-    let l:root = expand(get(g:, 'nuwiki_wiki_root', '~/vimwiki'))
+    let l:root = expand(l:w.root)
     " Ensure trailing slash to match what the original vimwiki returns.
     return l:root =~# '/$' ? l:root : l:root . '/'
 
-  elseif a:key ==# 'is_temporary_wiki'
-    return 0
-
   elseif a:key ==# 'ext' || a:key ==# 'extension'
-    let l:ext = get(g:, 'nuwiki_file_extension', '.wiki')
-    return l:ext[0] ==# '.' ? l:ext : '.' . l:ext
+    " wiki_list() already normalises the leading dot.
+    return l:w.ext
 
   elseif a:key ==# 'syntax'
-    return get(g:, 'nuwiki_syntax', 'vimwiki')
+    " wiki_list() doesn't carry `syntax`; read the per-wiki entry, then the
+    " global default.
+    let l:cfg = (exists('g:nuwiki_wikis') && len(g:nuwiki_wikis) > l:idx)
+          \ ? g:nuwiki_wikis[l:idx] : {}
+    return get(l:cfg, 'syntax', get(g:, 'nuwiki_syntax', 'vimwiki'))
 
   endif
   return ''